A transparent substrate such as glass or plastics, scatters transmitted light since fine water droplets are deposited on the surface of the substrate when the surface has a dew-point temperature or lower, thus leading to impairment in transparency, i.e. a state of “fogging”. As a method for preventing the fogging, heretofore, various methods such as the following (1) to (5) have been proposed.
(1) A method of treating the substrate surface with a surfactant to lower the surface tension (for example, Patent Document 1),
(2) a method of treating the substrate surface with a hydrophilic compound so that the substrate surface would be hydrophilic (for example, Patent Documents 2 and 3),
(3) a method of treating the substrate surface with a water-absorptive compound to lower the atmospheric humidity on the substrate surface (for example, Patent Document 4),
(4) a method of warming the substrate by providing e.g. a heater so that the substrate surface would be kept at a temperature higher than the dew-point temperature, and
(5) a method of treating the substrate surface with a water-repellent compound so that no fine water droplets would be deposited on the surface of the substrate.
Further, an effect of preventing the fogging (hereinafter, referred to as “anti-fogging performance”) has been required to be maintained over a long period under a working environment. Further, in order to maintain the anti-fogging performance over a long period under various environments, the durability such as abrasive resistance, water resistance, heat resistance, humidity resistance or water-wiping durability is also required.
However, each of the above methods (1) to (5) has difficulties as follows. In the case of the method (1), there is a difficulty in fixing a surfactant on the substrate surface and maintaining the low-surface tension over a long period. In the case of the method (2), e.g. a hydrophilic resin or a hydrophilic inorganic compound is used. However, in either case, especially inorganic soils are likely to be adsorbed and fixed, and it is difficult to maintain the hydrophilicity. In the case of the method (3), e.g. a water-absorptive resin is used. Though the water-absorptive resin is inferior in abrasive resistance or weather resistance to an inorganic compound, such a resin is excellent in an effect of preventing especially the inorganic soils and capable of readily maintaining the anti-fogging performance over a long period as long as the hydrophobic nature is imparted on the surface of the resin. In the case of the method (4), it is possible to maintain the anti-fogging performance semipermanently, but such a method is very expensive since energy accompanied by applying a current is always required. In the case of the method (5), in order to attain the anti-fogging performance, it is required to impart water-repellency to let even extremely fine water droplets with a diameter of at most 1 mm slip down or to prevent them from depositing, but such techniques are currently not present.
Accordingly, the present inventors considered that in order to readily maintain the anti-fogging performance at a low cost, it is possible to employ the method (3) in which a water-absorptive resin is used. As for the method in which a water-absorptive resin is used, an anti-fogging film and a coating agent to form a urethane resin having a surfactant, trialkanolamine and the like fixed, have been disclosed in order to obtain both anti-fogging performance and abrasive resistance (Patent Document 5).
In addition, Patent Document 6 discloses that it is possible to obtain a surface-coating composition for anti-fogging by reacting γ-aminopropyl trimethoxy silane and bisphenol A diglycidyl ether.    Patent Document 1: JP-A-2003-238207    Patent Document 2: JP-A-2001-356201    Patent Document 3: JP-A-2000-192021    Patent Document 4: JP-A-2002-53792    Patent Document 5: JP-A-2004-269851    Patent Document 6: JP-A-2002-161241